Browsing by Author "Kretzmann, Jared Eric"
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- ItemEvaluating the industrial application of non-destructive inspection of composites using transient thermography(Stellenbosch : Stellenbosch University, 2016-03) Kretzmann, Jared Eric; Venter, Gerhard; Schreve, Kristiaan; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: Transient thermography is a non-destructive testing method used in the detection and visualization of sub-surface flaws. Transient thermography could use one of two heating methods: step and square-pulse heating. Both these methods rely on observing the temperature rise of a surface that is subjected to a constant heat flux, while square pulse thermography also observes the subsequent thermal decay after the heat has been removed. The transient methods have not been thoroughly explored in literature with respect to the more popular methods, such as pulsed and lock-in thermography. Particular interest has been placed on investigating transient thermography on fiber-reinforced polymer (FRP) materials and its application in industry. Composites are prone to flaws such as delaminations, voids and inclusions that do not accurately represent flat-bottom holes, which are commonly evaluated in experimental work. Therefore, the inspection of thin artificial air-gaps and Teflon® delaminations were investigated. These artificial flaws can be considered to represent either a fully-separated or contacting delamination. A significant reduction in defect contrast and definition was observed for the thin delaminations, which is ascribed to the lower thermal resistance than that for flat-bottom holes. Further studies investigated the qualitative and quantitative performance of thermographic inspection on defective samples provided by an industrial partner. Experimental results demonstrated that variability in core geometry, ply arrangement, surface and sub-surface anomalies could be identiffied. The smallest detectable anomaly was found to be 1 mm wide, which was a spatial resolution limitation of the infrared camera. The investigated samples exhibited small radius and low resistance defects. It was found that current techniques to quantify defect depth are inadequate, especially if an accurate reference depth cannot be found. Thermography data is typically associated with subtle defect signatures that are strongly affected by non-uniform heating and surface variability. Advanced processing methods have been shown to help mitigate these effects. Various processing methods are reviewed from literature. Several methods were tested here for the first time, such as: multiscale retinex, matched filters, Markov error contrast and modified differential absolute contrast (IDAC) for step thermography. Transient thermography has shown to be a strong competitor amongst other thermographic methods for its simple application, relatively fast inspection times, and high thermal contrast for low defect resistance cases. It further enables the use of an entry-level infrared camera. The ndings of the artificial samples reported a maximum defect depth up to 7 mm was observed for clear Plexiglas®. The clear Plexiglas® can be considered to be the least optimal case of heating with optical excitation and has a low thermal emissivity. For the carbon and glass fibre reinforced polymers, a maximum detectable defect depth of 5 mm was observed, which is considered to be comparative or even better than pulsed thermography. The method was particularly better for low diffusivity materials, such as glass fibre composites.